1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to tunnel junctions for VCSELs.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics.
VCSELs include semiconductor active regions, which can be fabricated from a wide range of material systems, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Some VCSELs, particularly those used at long-wavelengths, incorporate tunnel junctions. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD).
FIG. 1 illustrates a typical long-wavelength VCSEL 10 having a tunnel junction. As shown, an n-doped InP substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the InP substrate 12, and an n-type graded-index InP lower spacer 18 is disposed over the lower mirror stack 16. An InGaAsP or AlInGaAs active region 20, usually having a number of quantum wells, is formed over the InP lower spacer 18. Over the active region 20 is a tunnel junction 28. Over the tunnel junction 28 is an n-type graded-index InP top spacer 22 and an n-type InP top mirror stack 24 (another DBR), which is disposed over the InP top spacer 22. Over the top mirror stack 24 is an n-type conduction layer 9, an n-type cap layer 8, and an n-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by forming an oxide layer. In any event, the insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40. When such a current confinement structure is adopted for a VCSEL, the tunnel junction 28 can also be placed above the current confinement structure (here, the insulating region 40).
In operation, an external bias causes an electrical current 21 to flow from the electrical contact 26 toward the electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that the current flows through the conductive central opening 42 and into the tunnel junction 28. The tunnel junction converts incoming electrons into holes that are injected into the active region 20. Some of the injected holes are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the conduction layer 9, the cap layer 8, an aperture 30 in electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical long-wavelength VCSEL having a tunnel junction, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate), different material systems can be used, operational details can be tuned for maximum performance, and additional structures and features can be added.
While generally successful, VCSELs similar to that illustrated in FIG. 1 have problems. One problem in realizing commercial quality long wavelength VCSELs, which is addressed by the tunnel junction 28, is optical loss. In long wavelength VCSELs, it is often critical to limit optical losses. To that end, p-doped materials, which absorb more light than n-doped materials, are replaced by n-doped materials and the tunnel junction 28. That junction converts electron currents into hole currents that are injected into the active region. That way, long wavelength VCSELs can be made with a non-absorbing n-type mirror on both the top and the bottom.
Tunnel junctions used in semiconductor lasers are thin (say 10 nanometer), reversed biased structures. Such tunnel junctions are usually n++/p++ structures in which the both sides have a high doping density using usually a low diffusivity dopant. This enables a low voltage drop across the tunnel junctions, and low free carrier absorption and sufficient free carriers in the semiconductor lasers. Dopants for such tunnel junctions should provide a high doping level in semiconductor matrix and should not diffuse around so as to maintain a sharp doping profile. Because of their low diffusivity, silicon and carbon are used for n-type and p-type dopants in tunnel junctions, respectively, especially for non-indium containing GaAs based VCSELs. For Indium-Alloy-based VCSELs, which are more pertinent to long-wavelength VCSELs, however, high doping density for p-type using carbon is, however, difficult to obtain. For example, p-type doping density of AlInAs lattice matched to InP with conventional doping techniques is less than 5×1019 cm−3, which is an amount of doping density for a good tunnel junction with low voltage drop. Thus, a new long wavelength VCSEL would be beneficial. Even more beneficial would be a new tunnel junction suitable for use in long wavelength VCSELs. Still more beneficial would be a new tunnel junction with a material pair that inherently possesses a large band-offset, which allows construction of good tunnel junctions without having to dope both sides as heavily as in the prior art.